1. Field of the Invention
The present invention relates to an optical pickup and an optical disk apparatus and, in particular, to an optical pickup for performing data recording and reproduction with respect to both CDs and DVDs.
2. Description of the Related Art
Conventional CD drives perform data recording and reproduction with respect to CDs, while conventional DVD drives perform data recording and reproduction with respect to DVDs. In addition to these drives, recently developed optical disk drives can perform data recording and reproduction with respect to both CDs and DVDs. Such drives have two light sources, a laser diode for irradiating a CD with a beam of focused laser light (a CD laser beam 9), and a laser diode for irradiating a DVD with a DVD laser beam 11. A typical drive has a common optical system for use in irradiating laser beams to either CDs or DVDs.
FIG. 6 shows a structure of a conventional optical pickup for use in an optical disk apparatus. The optical pickup has a CD laser diode 10 for irradiation of a CD laser beam 9 and a DVD laser diode 12 for irradiation of a DVD laser beam. The CD laser diode 9 and the DVD laser beam 11 are introduced into a composite prism 14 which comprises two prisms which are attached to each other via one surface. The composite prism 14 transmits the incident CD laser beam 9 through the common surface and reflects the light of the DVD laser beam 11 incident the common surface. The transmitted CD laser beam 9 and the reflected DVD laser beam 11 from the composite prism 14 then proceed coaxially until they are introduced into a subsequent collimator lens 16. The collimator lens 16 collimates the incident CD laser beam 9 and DVD laser beam 11 and directs them to a mirror 18. The mirror 18 then reflects the CD laser beam 9 and the DVD laser beam 11 into an object lens 20 which is arranged perpendicular to a paper surface. The object lens 20 converges the incident CD laser beam 9 and DVD laser beam 11 for projection onto the surface of a CD or DVD.
Common use of the composite prism 14, the collimator lens 16, the mirror 18, and the object lens 20 in both CD and DVD systems can advantageously reduce the number of components and associated cost.
Such a drive must produce a spot having a small diameter for the DVD system. For this purpose, a larger magnification power must be employed to ensure larger peripheral concentration of the light incident to the object lens 20. An optical magnification power 1/β is given as follows:1/β=(focal distance of the collimator lens)/(focal distance of the object lens 20)
Increasing the optical magnification power enables increasing the concentration of a laser beam along the periphery of the object lens, that is, RIM intensity. Accordingly, distribution of the concentration of a laser beam flattens the characteristics of the beam. Such characteristics enable further reduction of a spot diameter.
FIGS. 7A to 7C and FIGS. 8A to 8C show far field patterns, or FFP, of light incident to the object lens 20 and the associated concentration distribution of the laser beam for higher and lower optical magnification powers. Specifically, FIGS. 7A and 8A show positional relationships among the laser diode 10 or 12 serving as a light source, the collimator lens 16, the object lens 20, and a CD or DVD disk 22. FIGS. 7B and 8B show an FFP relative to an object lens pupil. FIGS. 7C and 8C show distribution of concentration (intensity) of a laser beam relative to an object lens pupil. Here, the concentration distribution of a laser beam is expressed as a relative concentration distribution with concentration at the center of the lens being one.
As shown in FIG. 7B, a smaller optical magnification power causes light incident to an object lens to have a smaller FFP, and causes a large deviation in concentration distribution characteristics of the incident laser beam within a range corresponding to the diameter of the objective lens pupil, as shown in FIG. 7C. Specifically, the concentration of the incident light is greater at its center than at its periphery, which results in a spot having a larger diameter formed on a disk plane through conversion by the object lens 20.
A larger optical magnification power, on the other hand, produces a larger FFP, as shown in FIG. 8B. This can ensure larger peripheral concentration (rim intensity), such as r1 increased from r2, which results in flatter characteristics of concentration distribution of a laser beam.
As described above, in a conventional drive in which the diameter of a spot for the DVD system is set small, a higher optical magnification power is employed for the common optical system so as to accommodate the DVD system, such as five to six power for a DVD system as opposed to three to four power for a CD system. Further, such a conventional drive employs a longer focal distance for the collimator lens 16. However, increase of the focal distance of the collimator lens 16 results in increase of a distance between the DVD laser diode 12 serving as a light source and the collimator lens 16. This can cause problems such as enlargement of the entire size of the optical pickup.
Still further, because a CD system requires higher coupling efficiency for recording using a CD-R/RW, it is necessary to provide a coupling lens 13 on an optical path, as shown in FIG. 6, to correct the magnification power. This leads to additional disadvantages such as an increase in the number of components.
Alternatively, a correction filter may be provided on the optical path between the laser diode 12 and the composite prism 14 to therewith attain flatter characteristics of the concentration distribution of the DVD laser beam 11. Provision of such a filter allows use of a smaller magnification power, such as is appropriate for the CD system, for the common optical system and a shorter focal distance of the collimator lens 16 to thereby reduce a distance between the laser diode 12 and the collimator lens 16. This arrangement, however, is disadvantageous in that, for example, addition of a new optical component, that is, a filter, and increase of relevant costs.
Meanwhile, Japanese Patent Laid-open Publication No 2001-134972 discloses a structure in which a diffraction grating is provided in a window of a semiconductor laser module to therewith correct the characteristics of the concentration distribution of a laser beam. This arrangement, however, disadvantageously requires a dedicated semiconductor laser module which has a diffraction grating installed in the window, and leads to an increase in costs.